All the code used to manage the mother boards will be
common to soon to come CPU board.

Move all that code into common.c

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>

On cmpc885 board, the ethernet addresses are stored in an
EEPROM that is accessed through SPI.

A 3 bytes command is sent to the chip then the content
gets read. At the time being a single block access is
performed, ignoring the first 3 bytes read.

Reword the SPI transfer to first send 3 bytes then
receive the content of the EEPROM so that there don't be
3 dummy bytes at the beginning of the buffer.

And move the function into common.c so that it can be
reused by the board that will be added in a future patch.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>

In preparation of the new cssi board called cmpcpro which
we be introduce in a future patch, move common
functions into a dedicated file in a common directory.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>